The Search for Environmentally Friendly Energy: Ethanol vs. Butanol

This post is contributed by Joseph Grimaldi (RPI)

An isobutanol molecule (Wikipedia)

The growing demand for carbon-neutral energy sources has fueled the search for biologically derived products. While the predominant liquid fuel is gasoline¹ the search is on for biofuel alternatives, but many pose considerable challenges. Currently ethanol is the most widely used biologically generated fuel. Ethanol, however, is difficult to transport in pipelines and also contains just 70% of the energy content of gasoline.² The United States has over 95,000 miles of pipelines and more than 160,000 filling stations with storage tanks. Hence, transportation and storage are crucial factors when considering which biologically derived fuels could suitably replace gasoline.¹

Ethanol

One of the challenges facing ethanol as a possible alternative is that it has a low density and is hydroscopic, so it cannot be easily piped.³ Another problem is that the production of ethanol is based on a limited number of agricultural resources, such as corn and sugar cane.¹ Development of a product that can be produced from biomass such as lignocellulose would greatly expand how much biofuel can be generated.² Cellulose reactant-based systems present problems as well. These systems are primarily xylose-based and therefore more difficult to convert to alcohols.¹ Ethanol is also costly to distill, which removes an amount of energy that ethanol provides versus the amount energy needed to produce it.³

Butanol

An alternative to ethanol which addresses transport and energy concerns is n-butanol or iso-butanol. Butanol has about 1.5 times more volumetric energy content than ethanol.^{2, 4} Another advantage over ethanol is that butanol has decreased water solubility; therefore, butanol can be both an additive to fuel and a standalone fuel source.² As a fuel source, butanol can be used in conventional engines², which is a considerable advantage because its use would not require specialized vehicles. Butanol is more hydrophobic then ethanol,³ therefore making it compatible with the existing fuel infrastructure and pipelines.^{1, 3} The largest concern facing alcohol-based biofuels is the ability to develop an efficient high-yield, commercially available process of production.¹

Do you think butanol is a good candidate for replacing gasoline?

References
1. Fischer, C.; Kleinmarcuschamer, D.; Stephanopoulos, G., Selection and optimization of microbial hosts for biofuels production. Metabolic Engineering 2008, 10 (6), 295-304.
2. Lee, S.; Chou, H.; Ham, T.; Lee, T.; Keasling, J., Metabolic engineering of microorganisms for biofuels production: from bugs to synthetic biology to fuels. Current Opinion in Biotechnology 2008, 19 (6), 556-563.
3. Steen, E. J.; Chan, R.; Prasad, N.; Myers, S.; Petzold, C. J.; Redding, A.; Ouellet, M.; Keasling, J. D., Metabolic engineering of Saccharomyces cerevisiae for the production of n-butanol. Microbial Cell Factories 2008, 7 (1), 36.
4. Inui, M.; Suda, M.; Kimura, S.; Yasuda, K.; Suzuki, H.; Toda, H.; Yamamoto, S.; Okino, S.; Suzuki, N.; Yukawa, H., Expression of Clostridium acetobutylicum butanol synthetic genes in Escherichia coli. Applied Microbiology and Biotechnology 2007, 77 (6), 1305-1316.